This invention relates to electronic musical instruments, and, more particularly, to a voicing system for an electronic organ.
Currently, the voices representing the different stops of an electronic organ are produced from pulse signals of a particular waveform, usually square wave pulses generated by a single tone generator system. By converting the square wave pulses to electrical signals having other wave shapes by various kinds of filtering and/or by selective combination of signals of various waveforms, signals are obtained which, when reproduced by a loudspeaker, produce sounds reasonably simulative of the different stops. Such systems have certain disadvantages and present design difficulties which more often than not result in compromises, the nature and severity of which will be appreciated from a brief review of the historical development and present state of the art of electronic organs.
Early in the development of electronic organs, it was commonly considered most desirable to use sawtooth waveform signals because they include all harmonics of the fundamental frequency up to a high order, albeit diminishing in amplitude in inverse proportion to the order of the harmonic. The tone quality of an organ voice, to the degree that it is determined by the harmonic structure of the tone, is determined by the harmonics present and their relative amplitudes. For example, the sounds produced by the family of "stopped" organ pipes contain odd order harmonics only, whereas the sounds produced by "open" organ pipes contain both odd and even order harmonics. There being no simple filtering technique for removing the even order harmonics from a sawtooth waveform signal, it was difficult, if not impossible, to produce from a sawtooth waveform signal a signal representative of a "stopped" organ pipe until Winston Kock taught in U.S. Pat. No. 2,233,948 (1941) a system of combining two sawtooth waveform signals, the frequency of the second harmonic of one of which is twice the fundamental frequency of the other, by inverting the phase of the higher frequency signal and combining the higher frequency signal at half amplitude with the lower frequency signal, thereby to cancel out the even harmonics of the lower frequency signal. This technique, known as "outphasing", enabled the derivation from sawtooth signals of voicing signals containing the only odd-order harmonics, and organ systems in which the tone generator signals were of sawtooth waveform, were manufactured and sold for some time.
More recently, tone generators for electronic organs are almost universally of the type that produce a square wave signal because of the simplicity and correspondingly lower cost of using digital techniques to derive from a single, or relatively small number of, master clock oscillators square waves having frequencies representing the tones of different octaves. However, a square wave signal has only odd harmonics, and produces a very hollow sound when acoustically reproduced, and since the clarinet is the only orchestral instrument whose sound signal has predominantly odd harmonics, it has been necessary to derive sawtooth waveform signals from the square wave signals by synthesis in order to have available signals containing both even and odd harmonics required to produce most organ voices. A synthesis technique known as "stairstepping", which is essentially the reciprocal of the outphasing technique taught by Knock, is described in Langer U.S. Pat. No. 2,533,821 (1950) and consists of adding in the correct proportions phase-locked square wave signals (which contain only odd harmonics the amplitudes of which are inversely proportional to the harmonic order) of a fundamental frequency, twice the fundamental frequency, four times the fundamental frequency, and so on, to produce a "stepped" waveform which, if it has enough "steps", is musically equivalent to a sawtooth waveform. It has been found in practice that for most purposes a stairstep wave having three steps (i.e., a combination of fundamental, the second harmonic at half amplitude and the fourth harmonic at one-fourth amplitude) is musically acceptable, the even harmonics falling in in substantially the ratios in which they would appear in a sawtooth wave.
Thus, most electronic organs today are based on the use of square wave signal generators and the selective combination of such square wave signals by the Langer synthesis technique to derive signals having the desired harmonic content of a sawtooth waveform signal. Filters of various types, such as low-pass, high-pass, band-pass, or combinations of these, are used to modify the sawtooth or square wave signals, as the case may be, to produce signals having other waveforms appropriate to the organ voice it is desired to simulate. Flute and clarinet tones are derived by suitably filtering the synthesized sawtooth waveform signal, and within these two broad families, the other voices are derived by suitable filtering and combination of the modified signals. In a very complicated organ, a separate filter could be provided for each note, each being tailored to alter the square wave signal in just the right way for its note, but because of the complexity and attendant high cost of this approach, it is much more common to go to the other extreme and provide a signal filter per organ voice for the entire range of the keyboard. Obviously, in a system in which all of the square wave signals corresponding to the keys played at a given time are mass processed by a single filter, the filter is necessarily a compromise in that it will have a different effect on the waveform of tones in the lowest octave than it will have on higher frequency tones; in spite of the necessary compromise, however, this approach is acceptable for many purposes and is utilized in many modern organ systems.
A primary problem inherent in filters commonly used in organs, be they low-pass, high pass, band-pass, or of other types, is that over the range of frequencies encountered in an organ having sixty-one notes, or forty-four notes in smaller organs, there is an upsetting of the scaling of a given stop because anything that affects the harmonic partials of the lowest key on the keyboard would also have an effect on the fundamental frequency of tones in the next higher octave. That is, if a filter were selected to attenuate the second harmonic of note C.sub.1, since the fundamental of C.sub.2 is the same frequency of the second harmonic of C.sub.1, the filter would have the same effect on the fundamental of note C.sub.2, and so on up the keyboard. There is no way to avoid compromise in this kind of system. For example, if one were to attempt to change a sawtooth signal into a waveform such as would produce a diapason sound on the one hand, or into a flute on the other hand, which requires even more severe attenuation of the harmonics, or if one were to attempt to change the sawtooth signal so that the resulting sound is brighter, like string or reed tones, the sawtooth signal must be warped rather drastically; thus, if one attempts to use a common filter to drastically warp the sawtooth signal into a number of different voices and still permit either the selective or simultaneous play of a string stop with the flute, or with the diapason, for example, something must suffer. If one goes up the scale, the flute tones will fall off in intensity and the string tones at the same time become louder. While there are ways to minimize these effects, such as by dividing the notes into small groups and applying separate filters to each group, or by prescaling or adjusting the amplitudes of the notes to preemphasize in some cases the higher notes so that when subjected to filters of the lowpass type which roll off the harmonics, the scaling would be brought back closer to what it should be with less severe distortion, obviously these "fixes" add to the complexity and cost of the voicing system.
The specific nature of the problems introduced when a single filter per voice is used for the entire keyboard range of frequencies will be better seen from an analysis of FIG. 1 which illustrates the normal connection of filter networks commonly used for modifying a sawtooth waveform input signal to produce signals which upon reproduction simulate common organ voices. The sawtooth signal applied at input terminal 10 is applied to the input of each of four parallel-connected filter circuits each of which includes a stop switch for connecting the output signal from the filter to an output terminal 12. Although technically not a filter, the uppermost parallel-connected path consists of a resistor 14 which, upon closure of a stop switch 16 marked CELLO attenuates by a predetermined amount and couples to the output terminal 12 a sawtooth signal corresponding in frequency to the note being played. The next filter is of the low-pass type and includes series-connected resistors 18 and 20 and a capacitor 22 connected between the junction of the resistors and ground. This type of filter attenuates those partials having frequencies where the reactance of the capacitor is low compared to the resistance of the resistors so that above some cutoff frequency there will be a gradual decrease in the amplitude of the higher order harmonics. The rolloff is very gradual at frequencies slightly above the cutoff frequency, ultimately reaching a point at which the rolloff is 6 dB per octave. By proper selection of component values, this low-pass filter modifies the sawtooth waveform input signal such that the resulting waveform when coupled to the reproducing equipment by closure of stop switch 24 produces the PRINCIPAL organ voice. The next filter, which may be called a DIAPASON filter, is a two-stage, low-pass filter including series-connected resistors 26, 28 and 30, a capacitor 32 connected to ground from the junction of resistors 26 and 28 and a capacitor 34 connected from the junction of resistors 28 and 30 to ground. Its operation is similar to that of the described one-stage, low-pass filter except that at frequencies substantially above the cutoff of the two cascaded stages, its attenuation is 12 dB per octave. The nature of the filter is such that it has a very gradual rolloff, with the knee of its characteristic set by the relative values of the resistors and capacitors; this is desirable when one is seeking to produce a diapason tone. Since organ voicing is a very subjective art, the relative values of resistors and capacitors are normally adjusted until the desired sound is obtained which might result in the knee of both stages being at the same frequency, or they might happen to be at different frequencies. The DIAPASON tone signal from the two-stage filter is coupled to the output terminal 12 by a stop switch 36. The fourth filter, which is conventionally used to produce reed sounds from a sawtooth waveform input signal is a high-pass filter including a capacitor 38 and a resistor 40 connected in series, the values of which are such that frequencies above the operating point of the filter are accentuated by up to 6 dB per octave. The output of this filter may be coupled to the output terminal 12 by closure of a REED stop switch 42.
Filters of the kinds shown in FIG. 1, which it is to be understood illustrate only a few of many different varieties utilized in electronic organs, are effective to more or less simulate the characteristics of the intended organ voices and are widely used in simpler organs. The filters selected for illustration do, however, serve to point up a difficulty that has plagued designers of electronic organs for many years, namely, that not only does the response of each of the filters (except the cello filter) vary with frequency, but each shifts the phases of the harmonic partials; at frequencies at which a filtering effect at the rate of 6 dB per octave per stage occurs, the phase of whatever signal is being transmitted is shifted by 90.degree.. In other words, each RC stage, whether in a low-pass or high-pass configuration, is capable of introducing a phase shift of up to 90.degree., and it will produce substantially a 90.degree. phase shift for all frequencies at which the filter produces a 6 dB per octave filtering effect. In order not to upset the scaling to a degree that the system cannot be used, these filters of necessity are designed to become effective at frequencies somewhere near the middle of the audio frequency spectrum; if their cutoff were set at a point so as to have the filter influence the low-order harmonic partials of the lower notes on the keyboard, all of the partials, including the fundamental, of the highest notes on the keyboard would be wiped out and the resulting signal unusable. The result of this compromise is that all tones at the lower end of the keyboard tend to be too bright by reason of the lower order harmonics being too strong in the case of the low-pass filters, and thus not nearly as effective as one would like.
The consequences of the phase shifts introduced by the filters become particularly serious when more than one stop is played at the same time, which, of course, is more often than not the case in electronic organs, since the effect of a given filter on the phase of any particular partial of any given stop is likely to be random and unpredictable because the cutoff frequencies of the filter as compared to the frequency of the fundamental of the note being played at a given time will vary depending upon the key being played. Obviously, a two-stage, low-pass filter will produce a rather drastic phase shift of most of the partials of the upper notes of the keyboard, the cello filter (a simple resistor) produces no phase shift, and the phase shift of the high-pass reed filter will be in the opposite direction from the phase shift of the low-pass filters, so that when a combination of stops is played, some of the partials will be additive and others will be subtractive, with the consequence that several voices no longer have their desired characteristics after being combined.
It is evident from the foregoing that there has been a long-standing need for an organ-voicing system which is capable of deriving from a simple waveform signal, such as the square wave signal from a commonly-used tone generator, signals having waveforms more amenable to filtering to produce signals representative of different organ stops which will retain their natural sound when two or more organ stops are played simultaneously. Among the objects of the present invention, therefore, is to provide such an organ-voicing system. A more specific object of the invention is to provide in an organ-voicing system that utilizes square wave signals as primary tone signals, a method and apparatus for deriving therefrom pulse signals of other wave shapes which with less drastic filtering produce tonally better organ voices, and at the same time greatly minimize the improper addition and/or subtraction of partials when two or more voices are played simultaneously.